Mechanical loading and physical exercise hold promise for enhancing bone mass and morphology. However, general exercise has proven ineffective in this realm, in large part due to the incomplete understanding of how mechanotransduction functions within bone. The long-term focus of this research is to better understand mechanotransduction and its age-related decline, in order to design novel mechanical stimuli that are capable of enhancing bone mass and strength even in the elderly. The rationale for this R01 application emerged from an exploratory project (R21AR053596) where we explored bone mechanotransduction using a novel technique called agent based modeling (ABM). In the course of the R21 project, we developed a series of ABMs that examined how real-time Ca2+ signaling/NFAT pathway activation induced acutely by mechanical stimuli influences bone formation and the degraded response of bone at senescence. In this R01 application, we will examine the hypothesis that age-related decline in bone formation induced by mechanical loading arises primarily via deficits in activation of the Ca2+/NFAT pathway. Through four specific aims, we will test our hypothesis using an integrated approach involving ABM simulations and in vivo animals studies that will incorporate assays for gene expression, cell function and bone formation. In our aims, we will quantify agerelated alterations in gene expression downstream of the Ca2+/NFAT pathway and their relation to deficits in cell function and bone formation. We will also demonstrate the requirement for this pathway in bone mechanotransduction by examining the impact of inhibiting (using NFATc1 knock-out mice and high dose Cyclosporine A; CsA) or enhancing its activation (using low-dose CsA). These experimental data will in-turn be used to develop multi-scale ABM simulations for how modulating activation of the Ca2+/NFAT pathway influences the dynamics of cell function and bone formation in young adult and aged animals. In the final S. Aim, the validated ABMs will be used to optimize activation of the Ca2+/NFAT pathway with the objective of restoring bone response to loading in the aged skeleton to levels observed in the young adult skeleton. Success of this project would clarify the Ca2+/NFAT pathway as a critical mechanism underlying the agerelated degradation in bone's ability to respond to physical stimuli, and will demonstrate the benefits that are anticipated via interventions in this pathway. Given that Cyclosporine A (one of the proposed interventions) is currently approved for clinical use, the feasibility of using physical exercise in combination with this drug in the elderly could readily be examined. Success of this project will also provide the clear rationale for exploration of interacting pathways (e.g., Wnt, MAPK) and mechanisms that are downstream of Ca2+/NFAT signaling (i.e., at the gene and protein expression levels) that define bone mechanotransduction in finer detail. Ultimately, such an improved understanding could enable the tailored design of novel interventions, not currently available, that powerfully counteract bone loss accrued over age. PUBLIC HEALTH RELEVANCE: The focus of this project is to explore deficits and interventions to signaling pathways underlying age-related degradation of bone mechanotransduction. Clinically, success of this project would provide the rationale for examination of this concept to augment bone mass in the elderly.